Inkjet hardcopy devices, in the following simply called printers, print dots by ejecting very small drops of ink onto the print medium. They may include a movable carriage that supports one or more printheads each having ink ejecting ink ejection elements. Recent printer designs include page-wide printheads. The ink ejection elements are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
A thermal inkjet printhead (e.g., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle and has a firing resistor located in the chamber. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink is fed from an ink reservoir integral to the printhead or an “off-axis” ink reservoir which feeds ink to the printhead via tubes or ducts connecting the printhead and reservoir, and is then fed to the various vaporization chambers.
Thermal inkjet printheads require an electrical drive pulse in order to eject a drop of ink. The voltage amplitude, shape and width of the pulse affect the printheads performance. It is desirable to operate the printhead using pulses that deliver a specified amount of energy. The energy delivered depends on the pulse characteristics (width, amplitude, shape), as well as the resistance of the printhead.
A thermal inkjet printhead requires a certain minimum energy to fire ink drops of the proper volume (herein called the turn-on energy). Turn-on energy can be different for different printhead designs, and in fact varies among different samples of a given printhead design as a result of manufacturing tolerances. Different kinds of tolerances add to the uncertainty how much energy is being delivered to any given printhead. Therefore, it is necessary to deliver more energy to the average printhead than is required to fire it (called “over-energy”) in order to allow for this uncertainty. As a result, thermal inkjet printers are configured to provide a fixed ink firing energy that is greater than the expected lowest turn-on energy for the printhead cartridges it can accommodate.
The energy applied to a firing resistor affects performance, durability and efficiency. It is well known that the firing energy must be above a certain firing threshold to cause a vapor bubble to nucleate. Above this firing threshold is a transitional range where increasing the firing energy increases the volume of ink expelled. Above this transitional range, there is a higher optimal range where drop volumes do not increase with increasing firing energy. In this optimal range above the optimal firing threshold drop volumes are stable even with moderate firing energy variations. Since, variations in drop volume cause disuniformities in printed output, it is in this optimal range that printing ideally takes place. As energy levels increase in this optimal range, uniformity is not compromised, but energy is wasted and the printhead is prematurely aged due to excessive heating and ink residue build-up.
In typical inkjet printers, as each droplet of ink is ejected from the printhead, some of the heat used to vaporize the ink driving the droplet is retained within the printhead and for high flow rates, conduction can heat the ink near the substrate. These actions can overheat the printhead, which can degrade print quality, cause the ink ejection elements to misfire, or can cause the printhead to stop firing completely. Printhead overheating compromises the inkjet printing process and limits high throughput printing. In addition, current inkjet printheads do not have the ability to make their own firing and timing decisions because they are controlled by remote devices. Consequently, it is difficult to efficiently control important thermal and energy aspects of the printhead.
Traditional printhead calibrations are done at the print head manufacturing lines and the calibration values are stored in the print head. This kind of calibration does not account for ink lot manufacturing variations, nor printhead to printhead variations. It only uses information from printhead manufacturing lot and ink color/type and is not be changed during printer operation.
Therefore, is a need for efficient thermal and energy control of the printhead in a printer.